Rename xml_create.py to xml_howto.py
Correctly ignore stuff.
Add carriage returns after text elements.
Added remove_attribute() method.

The remove_attribute() method allows removing of attributes from an XML element. Now the user has full control about adding, changing and removing attributes.
Review documentation.
Add initial example script.

This script creates a simple HTML document using GammaLib's xml module.
Remove unnecessary import.
Update ignore file after adding of examples.